Abdominal Pulsatile mass, AAA ruled out, what could it be?

Patient: During a routine physical exam my MD found a pulsatile mass in my abdomen. MD ordered an ultrasound to rule out an aortic anyeurism. Results were negative but pulsatile mass was detected during ultrasound. Full report stated that all of my organs were “normal” sized and that their was no presenting issue. MD believed that the mass is nothing to worry about and “is probably there because you are thin.” I have been having some upper abdominal pain, similar to indigestion. I am a 24 year old female, in nearly perfect health. Should I have a specialist try to determine the cause of the abdominal pulsatile mass or is it likely that the ultrasound ruled out any serious conditions, like cancer? Thank you.

Doctor: